1. Field
This application relates generally to wireless communication and more specifically, but not exclusively, to improving access terminal mobility.
2. Introduction
A wireless communication network is deployed over a defined geographical area to provide various types of services (e.g., voice, data, multimedia services, etc.) to users within that geographical area. In a typical implementation, access points (e.g., corresponding to different cells or sectors) are distributed throughout a network to provide wireless connectivity for access terminals (e.g., cell phones) that are operating within the geographical area served by the network. In general, at a given point in time, the access terminal will be served by a given one of these access points. As the access terminal roams throughout this geographical area, the access terminal may move away from its serving access point and move closer to another access point. In this case, the access terminal may be handed-over from its serving access point to be served by the other access point to maintain mobility for the access terminal.
An example of how the serving access point for an access terminal may be changed follows. The access terminal may regularly perform radio frequency (“RF”) measurements and determine that the signals being received from a neighbor access point (e.g., a so-called target access point) are stronger than the signals being received from the current serving access point by a certain margin. As a result, the access terminal sends a measurement report with this information to the network (e.g., to the serving access point). The serving access point then performs backhaul communication with the target access point to negotiate resources for the access terminal on the target access point. In addition, the serving access point sends a handover command to the access terminal, wherein the handover command identifies the resources assigned to the access terminal on the target access point. Finally, the access terminal connects to the target access point using these resources. A potential drawback of this handover procedure is that the target access point is not identified until the target access point is already stronger than the serving access point and, in some cases, the radio link with the serving access point may already be deteriorating at this point. In these cases, by the time the handover procedure starts, the signals from the serving access point may by too weak to be reliably received at the access terminal. Consequently, a radio link failure may occur whereby one or more of the above handover-related message may not be communicated between the access terminal and the serving access point.
In some cases, it may be determined that a particular neighbor access point is a source of mobility problems. For example, an excessively number of “late handovers” may occur in which an access terminal experiences radio link failure and attempts recovery on that neighbor access point before the source access point is able to prepare the neighbor access point. Such a situation may indicate that the neighbor access point is being measured as “too weak” relative to the serving access point. That is, the signals from the neighbor access point are not meeting measurement thresholds that would trigger the serving access point to prepare the neighbor access point for handover, or the signals are not meeting these thresholds until the access terminal is already experiencing radio conditions (relative to the serving access point) that are too poor for handoff mechanisms to work reliably. An access point could attempt to control this problem on a per-neighbor access point basis through the use of an access point-specific measurement threshold offset. However, the use of such an offset may not affect the access terminal's criteria for declaring radio link failure (e.g., initiating mobility towards a target access point).